Flexographic printing is a process of rotary letterpress printing using flexible printing plates and fast drying inks. In the inking section of a flexographic printing process, an arrangement of printing cylinders carries a predetermined amount of ink to the printing plates. The most common type of inking section contains two inking cylinders or rolls: a fountain cylinder and a transfer cylinder. The fountain cylinder, usually rubber covered, carries ink from the ink pan to the nip between the two inking cylinders. At the nip, ink is metered to the transfer cylinder and carried by it to the plate or plates mounted on the plate cylinder. Depending upon the stock to be printed and the nature of the design, the transfer cylinder, also referred to as an anilox roll, may have a surface of rubber, chrome-plated smooth steel, chrome-plated etched steel, or ceramic-coated etched steel.
To fabricate a ceramic-coated cylinder, a cylinder is first undercoated with corrosion-resistant stainless steel. The cylinder is then topcoated with a fine ceramic in molten form via a plasma coating process. The final ceramic coating has a consistent density across the surface of the cylinder. After each coating is applied, the cylinder is ground to exact tolerances. Finally, ink-carrying cells are created by engraving the ceramic-coated surface with a high resolution laser to form a variety of patterns, volumes and screen lines typically having a length, width and depth in the range of 10-40 microns.
After use in a flexographing printing process, the ceramic-coated surface of the printing cylinder becomes embedded with the residue of dried printing ink. This embedded printing ink residue interferes with consistent transfer of a predetermined volume of printing ink and consequently reduces the high resolution and quality of the final printing.
In addition to the dried printing ink residue, the ceramic-coated surface is also damaged by metal shavings which become embedded in the surface and which interfere with the quality of the final printing. These metal shavings result from the excessive shearing of the metal doctor blade used to control the amount of printing ink delivered to the printing plates.
Organic solvent cleaning methods utilizing, for example, acetone, methyl ethyl ketone and trichloroethane and various alcohols, have been used to remove the printing ink residue from the ceramic-coated surface of the printing cylinder. These methods introduce health, safety and environmental concerns, particularly in the disposal of the spent organic solvent. In addition, they are somewhat inefficient in removing dried printing ink residue from the ceramic-coated surface.
Ultrasonic cleaning methods, such as those described in U.S. Pat. No. 5,291,827, have also been used to clean the dried printing ink residue embedded in the ceramic-coated surface of the printing cylinder by shocking the surface with sound vibrations of a frequency greater than about 20 kHz for about 5-30 minutes. However, ultrasonic cleaning methods are problematic because they weaken and eventually crack the ceramic-coated surface of the printing cylinder, especially after repeated cleanings.